The present invention relates in general to automatic gain control (AGC) in a radio receiver, and more specifically to providing automatic gain control with a faster response to rapidly changing signal strength conditions associated with a mobile receiver moving into an underpass.
Automatic gain control (AGC) is a well known technique for maintaining a substantially constant average level of an amplified output signal even though the average input signal level is varying. In broadcast communication receivers, such as an AM receiver, AGC is employed to provide a consistent audio output level despite fluctuations in signal strength of a tuned-in broadcast signal or fluctuations occurring when returning to other broadcast signals of different power levels or distances from the receiver.
An AGC circuit controls the gain of an amplifier stage in inverse proportion to a measured average signal level downstream of the amplifier stage in the receiver. In an AM receiver, the information signal is encoded by amplitude variations in a transmitted carrier signal. In a typical AGC scheme, the AM intermediate frequency (IF) signal is lowpass filtered with a long time constant to remove any audio information signal when determining an AGC control output signal. However, the lowpass filtering results in a long response time of the AGC control circuit to sudden variations in the average signal level.
Sudden variations in signal level may occur when an automotive radio receiver moves from an open environment into a tunnel, into an underpass, or under a viaduct, for example. Although the signal strength of the AM signal may quickly decrease, the AGC control circuit responds to the change only after a delay, resulting in a decreased audio output of the received station and an increased in noise during the delay (known as AGC undershoot). Likewise, when the vehicle reemerges from the tunnel or viaduct and the signal blockage is removed, an over-amplified output is produced (referred to as AGC overshoot).